This invention relates to a trench-digging machine of the kind known as a ditch-digging machine in agriculture and a trencher in civil engineering.
In the following description the word "trencher" is to be understood in its broadest sense and is used irrespective of the power of the engine of the machine. The term "trencher" therefore covers ditch-digging machines.
Of course the general idea on which machines of this kind are based derives from excavating machines.
Of course, the digging element of trenchers mainly comprises a support frame, a digging boom, a digging chain and means for removing the dirt.
Two kinds of machine are known for digging trenches in the ground.
In a first kind of trencher, which has the merit of simplicity, the digging boom pivots in a vertical plane around the main transmission shaft driving the digging chain.
The depth of operation is adjusted by the amount of this pivoting--i.e., by the angle of attack on the ground.
The digging boom can therefore be of simple construction; however, the design has a number of disadvantages, more particularly at shallow and medium digging depths, for in such cases the boom is at a very reduced angle to the horizontal and the place of contact between the digging chain and the ground is a long way behind the vehicle.
The machine as a whole is therefore unbalanced. Also, a considerable length of the digging chain is in engagement with the zone where the ground is being attacked, so that friction forces are considerable.
The digging chain therefore wears prematurely and there is substantial mechanical stressing both of the boom pivot and of the system for controlling digging depth.
There are other disadvantages, more particularly as regards control of digging depth, which is delicate because of the reactions of the ground surface on a long overhung lever arm.
Because of all these disadvantages the complete digging apparatus is of poor mechanical strength.
Also, the overhang of the boom leads to considerable sweeping action on curves which tends to accentuate the curves and makes operations difficult.
In another and more sophisticated kind of trencher the boom is secured to a deformable parallelogram, pivots in a vertical plane and engages the ground far behind the carrying vehicle.
This system provides the advantage of separate adjustment of the angle of boom attack and of digging depth, but it has the same disadvantages as the system hereinbefore described, inter alia as regards boom overhang.
Also, the parallelogram is of complex construction comprising numerous pivot spindles and reciprocating actuators. The pivots experience the resultant of the forces exerted on the boom and are therefore subject to severe fatigue. Also, the parallelogram is positioned very high above the ground when the trencher is required to dig a shallow or medium-depth trench.
Consequently, the centre of gravity of the parallelogram is further from the ground, thus helping to further unbalance the complete apparatus. The weakness due to the kind of movement-transmitting system used means that the construction has to be strengthened, so that the weight of the complete apparatus increases.